The present invention relates in general to an optical amplifier operating on the principle of optical parametric amplification or four-wave mixing optical amplification, and more particularly to an optical amplifier in which phase matching between signal light and pump light is easily achieved and, hence, effective optical amplification of the signal light can be obtained over a broad frequency band.
Optical amplifiers of the type in which the amplitude of the electric field of light is directly amplified are applicable to the following uses in the optical fiber transmission system:
By increasing the output of a light source of the signal light in an optical transmitter, the transmission distance can be increased. When the optical amplifier is used for the light source of local light in an optical receiver in a coherent optical wave communication system, the reception sensitivity can be improved.
By performing optical amplification in the stage immediately before the photoelectric conversion stage, the reception sensitivity can be improved.
By the direct amplification of light, as compared with the method in a conventional optical repeater in which a light signal is once photo-electrically converted into an electric signal and then the electric signal is amplified, it becomes possible to make the repeater itself smaller in size and also to increase the repeater-to-repeater distance.
There has been known an optical amplifier in which optical parametric amplification of signal light is achieved by nonlinear effect of second order obtained when signal light and pump light are propagated through an optical waveguide structure made of a nonlinear optical material.
There has also been known an optical amplifier in which four-wave mixing optical amplification of signal light is achieved by nonlinear effect of third order obtained when signal light and pump light are propagated through an optical waveguide structure made of a nonlinear optical material.
However, such conventional optical amplifiers have had a disadvantage that phase matching between the signal light and the pump light is not always easily achieved therein and, hence, effective optical amplification of the signal light is obtained only within a narrow frequency band.
From the U.S. Pat. No. 5,274,495 an optical amplifier is disclosed which is adapted such that signal light and pump light are propagated through an optical waveguide structure therein made of an optically nonlinear material to thereby achieve optical parametric amplification or four-wave mixing optical amplification of the signal light, and which is provided with means for attenuating idler light to be generated within the optical waveguide structure by adding special dopants to the fiber.
This allows an attenuation of the idler wave but cannot avoid a repowering of the idler wave during the pump process.
Optical parametric amplification is carried out by a power transfer from a pump wavelength towards a signal wavelength. This energy exchange depends on phase matching between the waves of the two wavelengths, on their power and on fiber nonlinear coefficient. For ‘small-signal’ e.g. signal with a small power, signal power increases linearly with fiber length. To be efficient the signal power must increase up to the level of the pump power, and than the energy exchange between the wave is reversed. In result the signal wave recharges power back to the pump wave. Then the signal power decreases with length, which makes amplification inefficient. In order to avoid signal power traveling back to the pump, fibers length in known optical parametric amplifiers is shorter than the length from which signal power decreases. Pump power remains non-depleted during amplification. The efficiency of parametric amplification strongly depends on the frequency shift between signal and pump (through phase matching) wave. Consequently signals with different wavelengths do not experience the same gain and for a given fiber length, some wavelengths are more amplified than the others. Finally the gain spectrum is not flat.
Accordingly, an object of the present invention is to provide an optical amplifier in which phase matching between the signal light and the pump light is easily achieved and, hence, effective optical amplification of the signal light can be obtained over a broad frequency band by an effective suppression of the idler wave.